regulator: max8998: Use arrays for specifying voltages in platform data

This patch modifies the platform data of max8998 to use arrays for
specifying predefined voltages of buck1 and buck2 instead of separate
field for each voltage.

This allows to simplify the code a bit and will help in adding support
for Device Tree, which will be introduced in further patch.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>